For soundproofing applications, they make acoustical putty that you put around the electrical box. However, what if you can't get behind the electrical box? In my case, I have a wall floating on hat channels about 3" from the studs. The electrical box I will be using is an old work box with a clamp that allows the box to be fastened to the drywall rather than studs. This means that I will not be able to get to the backside of the box to apply the putty. I though about putting it around the outside of the box before sliding it through, but that doesn't seem like it would be very stable.

This leads me to me idea of putting the putty on the inside. I uploaded pics of a mock application. The outlet fits well and I think there would be plenty of room for the wires. I know next to nothing about electrical, though, and I worry about not accounting for things that might not play well with the putty.

Does anyone advise against this? I hope not because it's really the only way I can do this.

Update:

Here's what I ended up doing. At first, I had a hard time understanding Evil Greebo's diagram, but I think this was the general idea he was conveying.

First, I cut out the smaller hole for my outlet. Then I cut out the entire section 2" around the outlet hole.

I installed some support boards on either side of the opening.

I used an old work box with adjustable flanges and installed it into the cut-out.

I then covered it with my acoustic putty (QuietPutty) and left an opening for the wires. Looks like some kind of alien organ, doesn't it?

I ran the wires into it and then packed the putty tightly around the wires for good measure. A flathead screwdriver worked great for packing and manipulating the putty.

Finally, I placed the section into the wall and screwed it into the support boards. My pic only shows two screws, but I actually screwed in all for corners.

If anyone else can think of a better way to do this, let me know because I am about to do it again, but this time with 5 outlets, one of which is a two ganger.

Can't quote code but generally no because you need ample space within the box for the wiring, and the putty significantly reduces the available volume of the box. Also - dude - what the heck are you building?!?!?
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The Evil GreeboDec 3 '12 at 21:19

If I compress it enough all throughout, I can get it to 1/8" thickness. Does losing that much area really have that much of an impact on required space? As to what I am building, many have referred to it as a "murder room".
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oscilatingcretinDec 3 '12 at 21:23

By "murder room", you mean a room with a locked door, so you have to find useful things to use so you can get out of it, right?
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jrgDec 3 '12 at 21:32

3

A flash point of 227C is very low and would be a potential hazard if a device in that box started arcing. Something like "Plasduct" electrical putty would be a better choice. Electrical putty is used in boxes and service enclosures to seal the openings around wires and is fireproof.
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shirlock homesDec 4 '12 at 11:20

1

The first problem could be not enough open space in the box for the wires and device. In this situation you'll count each conductor entering the box as 1 (1 hot + 1 neutral = 2), all grounding conductors in the box count as 1 (3). The device counts as 2, so your box fill is 5. If you're using #14 wire you'll multiply by 2 (for #12 multiply by 2.25), which will tell you how many cubic inches of space you need inside the box (10 Cu.in. in this case). The other problem you'll have, is proximity to combustible materials.
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Tester101♦Dec 4 '12 at 12:22

2 Answers
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The box / soundproofing should have been installed before drywalling. If you want to do it right, cut a piece of the drywall out to give you room to work and install it properly, then close up the wall again.

There are no studs directly behind the wall that a box can mount to, hence why I planned ahead with a few old work boxes. I think what I will have to do is cut out a hole for the box then cut 1 1/2" around the perimeter of it and remove the section. I'll then install the box, putty the back, place the section back in the wall, and use acoustic caulk to seal the perimeter. I would need some rigid backing on the other side so I can screw or glue the section in. Perhaps I could glue 1/2"-thick strips plywood around the hole on the other side. Any ideas?
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oscilatingcretinDec 5 '12 at 11:38

It'd be easier for you, I think, to just open up the wall enough to install 2x4's where you need them, connecting them to the existing studs.
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The Evil GreeboDec 5 '12 at 12:33

This is not an option since the whole point of floating the wall on furring channels is to decouple the wall from the studs. Attaching another stud to the existing studs and then mounting the box to that stud would short out the system by creating a direct sound path between my wall and the frame behind it.
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oscilatingcretinDec 5 '12 at 12:47

Open the drywall up enough that you can put a 1' 2x4 behind the wall. Mount the 2x4 to the drywall with a drywall screw placed every 3" along that 2x4. The first 2x4 should be completely covered by the edge of the drywall.

Take a SECOND length of drywall and mount it to the first one, so that it is exposed through the hole. Connect it to the first board using several screws.

Wrap your new box in acoustical putty and mount it to the 2nd board.

Close up the opening by attaching replacement drywall patch on one side to the exposed stud, and on the other to your hat channels.

Then you'll have support boards fixed to the silent side of your wall, and your box, wrapped in putty, will be connected to those boards.

And if this really is a murder room, I'm testifying against you in court! ;)
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The Evil GreeboDec 5 '12 at 13:15

I see what you're saying, but I think that mounting wing/flap/whatever is going to create a noticeable surface disparity when the drywall section is replaced. I have one of those boxes and that wing is about 1/16" thick. Perhaps I could mortise the new 2x4 so the wing will fit.
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oscilatingcretinDec 5 '12 at 13:27

If I am going to do that, though, I think I may as well just use my old work clamp-style box and go with my initial idea as commented in your first answer.
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oscilatingcretinDec 5 '12 at 13:32

Also, this is a double layer of 5/8" drywall, so I'd have to get an electrical box extender so the outlet can be flush to the wall's surface.
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oscilatingcretinDec 5 '12 at 13:36